It is generally known that, in order to manufacture an electrical cable, a process known as extrusion is used in which the electrical conductor is fed into a machine called the extruder which extrudes a plastics material on the conductor thereby creating an electrically insulated cable which may be used in various applications including the transmission of electrical energy and the transmission of signals, etc.
This process has advanced through the years, and electrical cables have been used in telecommunication applications in which many hundreds of cables are bunched together to form a trunk cable which is then installed between telephone contres, to form part of a telephone link network.
In order that the cable may be economically manufactured, various methods have been employed to reduce material costs which involve controlling the plastics coating of the cable to a minimum thickness. In order further to reduce material costs, a method has been introduced whereby the plastics coating of the cable is chemically "blown" or "foamed" producing a cellular structure which maintains the same wall thickness of insulation but, in fact, uses less plastics material. This method has been found to be successful, and the cellular cable produced maintins all the electrical characteristics required by the specifications relating to the final application, as well as being considerably cost effective in manufacture.
To control the coating process, all systems available so far, monitor the capacitance and/or diameter of the extruded cable and make appropriate adjustments when the measured values differ from those required.
These prior methods do not allow for the fact that diameter and capacitance are two conflicting parameters in terms of process control, and that the correction of one can drive the other ut of tolerance. In most cases capacitance control is the dominant correcting factor which results in an extremely slow overall correction process.